While I wasn't able to find schematics for how the Flexwrite is wired up to the KDF9 (say), there are some Flexwriter manuals availavle on bitsavers. The Flexwriter apparent came with 5 bit and 8 bit encodings (or "5-channel" and "8-channel" variants, as they call them).
The encodings (at least the earlier ones used for the KDF9) were Friden-specific, and neither ASCII nor compatible to anything else. However, the 5-bit encoding worked similar to other Teletext-like encodings (there are shift-states for upper and lower case, and the "all holes punched" encoding is a null encoding).
The Communications System and Equipment (1961) brochure suggest that serial communication was in use, also over longer distances. (Though that doesn't exclude the possibility the Flexwriters connected directly to the computer used a parallel connection).
There is no indication of any software flow control (XON/XOFF). There is also no indication of character codes which could be used to change channels etc. as part of the transmitted data, so I'd assume this could only be done on the flexwriter.
In various materials it is mentioned that the Flexwriters operated at a fixed speed.
There is also plenty of material on the KDF9. This here describes the I/O system as using DMA, that's probably why software flow control wasn't necessary.
It also mentions that there was a dedicated Flexwriter used as the console for the KDF9 which had an "attention" key that was hardwired as an interrupt source, and this feature seems to have been unique for this special Flexwriter. So that was probably a custom modifications.
All I/O channels of the KDF9 work similarly, so likely other Flexwriters were just hooked up to a single channels like the console.
Looking at the PDP-1 Maintenance Manual, it says on page 4-4
(c) The typewriter logic cable is equipped with a 50-pin Cannon connector
and on page 9-10 ff.
The PDP-l typewriter is an IBM Model B equipped with a Soroban electromechanical encoder
and decoder. The decoder contains six information solenoids. These solenoids are driven from
the typewriter control unit. The positions of the solenoid armatures are mechanically decoded
to determine the desired character. Besides providing the six information signals, typewriter
control also energizes the typewriter cam magnet, thereby causing the typewriter to print the
desired character. The typewriter decodes most control characters (carriage return, backspace;
space, tab, and shift), and all print characters, from the six information solenoids. The two
color characters, red and black, are decoded by the typewriter control unit, which directly
controls the color shift solenoid in the typewriter.
When the operator strikes a typewriter key, the typewriter encoder presents information signals
to the typewriter control unit. The encoder mechanically encodes all print characters into six
coded switch closures plus a common switch closure. Control characters are not encoded at
the typewriter. When a control character key is struck, a single switch is closed. Closure of
such a control char'acter switch is encoded within typewriter control.
So this looks very much like a parallel connection, very likely 5 bits plus parity.